1. Field of the Invention
The present invention relates to an X-ray CT scanner including a two-dimensional detector that has a plurality of detecting elements laid out two-dimensionally in the slice-thickness and channel directions thereof in a manner such that a plurality of segments having different slice thicknesses are formed in the slice-thickness direction for multi-slice scanning, the sensitivity distributions of the segments being uniformed in the slice-thickness direction. In the invention, the slice-thickness direction (or simply, slice direction or segment direction) corresponds to the direction of the rotation axis of a gantry of the scanner.
2. Description of the Related Art
X-ray CT scanners include a fan-beam (single-slice)X-ray CT scanner that has been adopted in the past.
The fan-beam X-ray CT scanner has an X-ray source and detector opposed to each other with a subject (for example, a patient) between them. The detector has detecting elements, which constitute, for example, approximately 1000 channels, arranged in the form of a sector in a (channel)direction orthogonal to a body-axial direction of the subject.
Though a variety of types of detector can be used as the X-ray detector, a scintillation detector, which is easily compacted, is frequently employed. The scintillation detector has a scintillator functioning as an X-ray detecting element (segment) and a photo sensor such as photodiode. X-rays transmitted through a subject are received by the scintillator, in which fluorescence is generated in response to the reception. The fluorescence is converted into an electric signal by the photo sensor. The electric signal constitutes X-ray transmittance data to be outputted from the detector segment by segment.
In the X-ray CT scanner, a fan-shaped X-ray beam is irradiated from the X-ray source to a certain slice plane (or, simply, a slice) set in the subject. An X-ray beam transmitted by the slice plane of the subject is detected by the detector, and then X-ray transmission data is acquired.
The acquired X-ray transmission data is sent to a data acquisition system (DAS) having data acquiring elements associated with the detecting elements of the detector. Each element carries out amplification or the like and acquires projection data (one data acquisition is referred to as one view).
While the X-ray source and detector are rotated in unison about the subject, X-rays are irradiated and data acquisition is repeated approximately 1000 times. Consequently, projection data in multiple directions of the subject is acquired. Based on the projection data in multiple directions, the image of the slice plane of the subject is reconstructed.
In such a single-slice X-ray CT scanner, the image of a certain slice plane of a subject is produced. It is therefore hard to produce images of a wide range of the subject for a short period of time. There is therefore an increasing demand from doctors and the like for producing high-definition (high-resolution) images of a wide range of a subject for a unit period of time.
In an effort to meet the demand, studies have been made on a multi-slice X-ray CT scanner in recent years.
The multi-slice X-ray CT scanner has a plurality of columns (a plurality of (N) segments) of detecting elements, each of which is the same as the one employed in the single-slice X-ray CT scanner, in the body-axis direction of a subject (also referred to as a slice-thickness direction or segment direction). The detecting elements constitute a two-dimensional detector having detecting elements numbering the product of M channels by N segments. In this case, elements of a DAS are associated with the detecting elements of the two-dimensional detector.
In other words, the multi-slice X-ray CT scanner has an X-ray source for bombarding a conical X-ray beam, and the foregoing two-dimensional detector. X-rays of the conical X-ray beam (diameter of an effective field of view, FOV) passing through a subject are detected by the two-dimensional detector, whereby projection data of multiple slice planes of the subject is acquired at a time. Thus, the multi-slice X-ray CT scanner is expected to enable acquisition of high-definition images from a wide range.
Various proposals have been made of the configurations of such a multi-slice X-ray CT scanner and two-dimensional detector employed in the multi-slice X-ray CT scanner.
For example, known is an idea of freely changing one slice thickness by combining X-ray data items detected by a plurality of segments through image post-processing based on detected data.
Thinking of the specifications of a two-dimensional detector and DAS for a multi-slice X-ray CT scanner, several parameters have significant meanings. To be more specific, for improving the resolution in a body-axis direction, it is necessary to finely set the pitches in the body-axis direction of elements corresponding to segments of the detector (slice thickness) relative to adjoining ones. For expanding a scanned region in the body-axis direction (for eventually shortening the scan tome of a certain region, the size of the whole detector (the number of columns corresponding to the segments of the detector) must be made larger. In an effort to clear both the requirements that are seemingly contradictory, that is, improvement of the resolution in the body-axis direction and expansion of a scanned region, it has been conceived that sufficiently small detecting elements that are fine divisions of a detector are arranged in the body-axis direction by the number of columns (segments) defining a sufficiently large size.
However, on the detector side, there are limitations in a minimum size of an element (in a slice-thickness direction) and a maximum number of elements because of the problems that geometrical efficiency is deteriorated with finer segmentation of the detector and that the density of wiring patterns increase with an increase in number of elements. It is therefore currently thought that approximately 1 mm and approximately 30 columns are feasible levels of the minimum size of an element and of the maximum number of columns of elements respectively.
For arranging approximately 30 columns of detecting elements, it is necessary to install a DAS having the number of elements corresponding to the number of segments or columns of the detecting elements. A simple countermeasure is to arrange a plurality of (30) columns of currently-employed DASs. In reality, there are limitations in the number of elements of a DAS that can be arranged because of the problem of preserving an installation space in a scanner system or the problem of ensuring appropriate cost performance. The existing high-density installation technology and manufacturing cost permit about 10 columns of elements as a level feasible in the near future.
Since restrictions are thus placed differently on the parameters such as the number of elements of a DAS, a minimum size of an element of a detector, and a maximum number of elements in the detector, it is hard to attain high resolution in the body-axis Direction and a wide scanned region by nonchalantly combining these parameters. A further commitment to novelties and improvements is requested.
On one hand, in the case that the scintillation type detector (i.e., solid detector) incorporating a combination of scintillators and photo sensors is used, light is absorbed at the edges in the slice-thickness direction of the scintillator. Due to this light absorption, know is that the sensitivity distribution of each scintillator is dropped at the edge portions in the slice-thickness direction of its X-ray incidence area. Understood from this fact is that sensitivities (amounts of output light per unit size) at the edge portions of a scintillator depend on its sizes (i.e., width in the slice-thickness). In other words, the thinner the slice thickness, the lower the entire sensitivity of a detector, because the lowered sensitivities at the edge portions of a scintillator having thinner slices have larger influence on the entire sensitivity.
Owing to this, when an X-ray detector is provided a plurality of detecting segments mutually different in slice thickness, irreguralities in sensitivity distributions of segments are brought about depending on differences in slice thicknesses, which will cause artifacts in reconstructed X-ray images.